1. Field of the Invention
The present invention relates to a film capacitor. More particularly, the present invention relates to an improvement of a wound film capacitor and a stacked film capacitor.
2. Discussion of Related Art
Conventionally, as one type of film capacitor for use in an electric device, there are known a wound film capacitor and a stacked film capacitor. As disclosed in JP-A-2007-220720, the wound film capacitor is generally produced by winding a multilayer film (metallized film) in which a vapor-deposited metal film (layer) as an electrode film is formed by a vacuum deposition method on one surface of a dielectric (layer) consisting of a resin film (layer) having insulating properties, which is formed of polypropylene and polyethylene terephthalate, for example, or by winding a plurality of basic elements which are stacked on one another. In addition, JP-A-2008-91605 discloses a wound film capacitor that is formed by winding a multilayer film including a dielectric made of a resin film and a vapor-deposited metal film formed on each surface of the dielectric, and another resin film including no vapor-deposited polymer film, while they are stacked with each other. On the other hand, as disclosed in Japanese Patent No. 3908094, for example, the stacked film capacitor is formed by stacking a plurality of above-described multilayer films and the resin film such that the resin film is positioned between the multilayer films. In brief, a wound film capacitor is formed by winding a dielectric (layer) including a resin film (layer) and a vapor-deposited metal film (layer) while they are stacked so as to be positioned alternately, and a stacked film capacitor is formed by stacking a dielectric (layer) including a resin film (layer) and a vapor-deposited metal film (layer) so as to be positioned alternately.
Recently, electric devices have been increasingly demanded to have smaller size and higher performance. Along with this demand, a film capacitor is also increasingly demanded to have smaller size and higher capacity. In the above-described stacked and wound film capacitors, in an effort to meet the demand, it is considered to simultaneously realize downsize of the film capacitor and improvement of capacity thereof, by reducing the thickness of a resin film constituting a dielectric layer to improve the dielectric constant of the resin film.
However, when the thickness of the resin film is reduced, influence of impurities (residue) in the film materials becomes bigger, resulting in deterioration of withstand voltage of the resin film. For that reason, there is a limitation on a reduction in thickness of the resin film. Thus, in the film capacitor having a conventional structure, only by reducing the thickness of the resin film, it was extremely difficult to reduce size thereof and increase capacity thereof (improve performance thereof) to the extent that the demanded requirements are sufficiently fulfilled, while securing high withstand voltage.
To increase the withstand voltage of the resin film, smoothness of the surface may be increased by reducing roughness of a surface of the resin film. The resin film used in the conventional film capacitor is, however, made of a stretched film. Thus, in such a resin film, it was not easy to obtain enough surface smoothness that can sufficiently improve the withstand voltage.